My company sells a module which allows you to import excel file with product feeds and image urls. It then goes to each image url and fetches the image from there and stores it in the db. Contact the sales here http://www.catchytech.com/Contact.aspx if you're interested.
The images are initially saved into database and the thumbnails/images are created on the fly.
I guess your image names reflect the Product Name or ID etc. You can create another application that will read all the images from a specific folder and dump them into DB one by one in the Nop_Picture table and set the image ID and product ID in the Nop_ProductPicture table accordingly.
I have written an sql script and stored proc that will upload the images into a temp table in the db that you can then use another bit of sql to match them to your products. Let me know if you want a copy.